NEW ORLEANS —  Federal scientists and BP say oil appears to have leaked last month from the drilling wreckage lying at the bottom of the Gulf of Mexico near where a BP well blew out in 2010, causing the nation’s worst offshore spill.

A probe started after a sheen was discovered Sept. 16 in the waters near the site indicates the oil may have seeped from a mile-long metal tube, called the riser, which connected the Deepwater Horizon drilling rig to the Macondo well.

The well itself, capped after more than 200 million gallons of oil spewed, is not believed to be leaking, officials said. The oil sheen posed no environmental threat because it was a small amount of oil and was far from land, federal officials said.

The announcement of oil leaking out comes at a sensitive time as BP and the Justice Department negotiate terms of a possible settlement to resolve government’s claims against the oil giant. Several billion dollars are at stake if the talks produce a settlement for what likely will be record-setting civil and criminal penalties.

The tests of the sheen showed chemical signatures of Macondo oil mixed with drilling muds, which are lubricants used both in drilling the well and trying to plug it after the April 20, 2010 explosion.

“This led to the conclusion that the oil causing the sheen out at the surface is likely coming from the riser. The riser has drilling muds in it,” said Frank Csulak, the scientific support coordinator for the National Oceanic and Atmospheric Administration.

He said at most the riser could contain about 75,600 gallons of oil, but experts expect less than that is trapped inside it. He said tests have ruled out the possibility that the new sheen came from oil sitting on the bottom of the Gulf because it was not weathered and degraded.

Csulak said BP and Transocean Ltd., the drilling company contracted by BP to drill the well, have been given until Friday to provide a plan of action. He said the companies could be asked to remove whatever oil is left in the riser.

“It would be possible to remove the oil; it’s not going to be easy,” he said. The amount of oil that leaked out is small and that once the oil reached the surface “it evaporates and dissipates within hours.”

He added that surveys hopefully would determine what’s going on deep down on the Gulf floor. “We have no idea if oil is coming out of one spot or multiple locations,” Csulak said. It’s unknown if it was a steady leak or intermittent.   

Still, the sheen and acknowledgement that BP’s oil is reaching the surface of the Gulf sparked a new flurry of condemnation of the oil giant and stoked claims that BP and the Coast Guard have been unwilling to investigate whether BP’s well was completely sealed in. Independent scientists and environmental activists have reported observing sheens around the Macondo well for more than a year.

On Thursday, U.S. Rep. Ed Markey, D-Mass., demanded that BP be forced to conduct new surveys of the well and the wreckage around it. He said BP should allow the public to view underwater video of its surveys in real-time.

“One can only hope that the nightmare well has not come back to haunt the people of the Gulf,” Markey said. “There is no room for error, and no room for obfuscation, when it comes to this matter.”

Brett Clanton, a BP spokesman, said the company had been working closely with federal agencies to investigate the sheen. He added that there was no indication that oil had leaked from the capped well.

He said tests showed the presence of alpha-olefins, a lubricant used in drilling mud and not found in oil coming directly from a reservoir. He said the presence of alpha-olefins “strongly suggests” the oil came from the wrecked riser.

Lou Colasuonno, a spokesman for Transocean, said the company would “rely on the lab analysis as to the origin of the oil.” But he suggested it was BP’s responsibility to deal with whatever oil remains in the riser. Transocean and BP have argued in court over responsibility for the spill.

Christopher Reddy, a chemical oceanographer at Woods Holes Oceanographic Institution, said the Coast Guard’s analysis was reliable but he stressed getting the companies to investigate the leak further was paramount.

“Is it a leaky faucet that we’ll have to deal with or things could get worse?” he said. “It is worth discovering.”